The present invention relates generally to industrial boilers and steam generators and more specifically to the prevention of calcium, magnesium and silicate scale build-up on the inside surfaces of the boilers and steam generators.
In the design and operation of steam generators, it is essential that the boiler be designed efficiently and operated efficiently as well. However, during operation, scale build-up on the inside surfaces of a boiler can be very problematic. The presence of scaling inside the boiler significantly interferes with the heat transfer from the inside surface of the boiler to the water or steam. By interfering with the heat transfer process, the scaling significantly reduces the efficiency of the boiler. Even more important than heat loss is that scaling causes overheating of the boiler metal and can result in subsequent tube failures. Costly repairs and boiler outages are the results of such a condition.
Also, scaling on the surface of metal leads to corrosion underneath the scaling. Thus, scaling contributes to corrosion of systems which prematurely weakens the structure and leads to leaks and ruptures in the boiler system. Therefore, scale prevention directly contributes to corrosion prevention as well.
In boilers used in steam generators, it has been found that the primary components of scale build-up are calcium derivatives, magnesium derivatives, silicates and iron derivatives in the feedwater. Deposits formed on the inside surfaces of the boiler can be removed by shutting the boiler down and cleaning the inside surfaces of the boiler, either chemically or mechanically. Obviously, any shut-down of a boiler is going to adversely effect the efficiency and overall economics of the operations.
Alternatively, treatment chemicals that inhibit or reduce scale buildup can be added directly to the boiler feedwater. Soda ash, sodium aluminate and phosphates have been such additives. These additives are non-corrosive to internal surfaces, but they can reduce heat transfer efficiency if they form insulating scales such as calcium phosphate scales. Chelating agents such as nitrilotriacetic acid and ethylenediamine tetra acetic acid offer superior cleanliness over phosphate programs. However, chelating agents have the potential for corrosion if they are overfed or fed to a system containing oxygen.
Synthetic organic polymers offer the cleanliness of chelates without the potential for boiler corrosion, even if overfed. They also offer greater hardness transport than conventional phosphate programs. However, until now, such synthetic organic polymers have been limited to non-biodegradable polymers. Increasing pressure on the water treatment industry has generated interest in the development of environmentally friendlier chemistries that offer biodegradability in addition to improved scale inhibition.